In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is positive-working. Conversely, if the unimaged regions are removed, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Two types of screening are used in lithographic printing. In conventional halftone screening, sometimes called Amplitude Modulation (AM) screening, the halftone dots that together simulate the original image are arranged on a fixed geometric grid. By varying the size of the halftone dots the image can be simulated. In stochastic screening, sometimes called Frequency Modulated (FM) screening, the dots are same size but the number of dots and their location is varied to simulate the original image. The concept of stochastic screening is that halftone dots placed randomly, instead of in a fixed pattern, will not cause moiré patterns.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, mufti-layer elements are disclosed, for example, in Shimazu, U.S. Pat. No. 6,294,311, U.S. Pat. No. 6,352,812, and U.S. Pat. No. 6,593,055; Patel, U.S. Pat. No. 6,352,811; and Savariar-Hauck, U.S. Pat. No. 6,358,669, and U.S. Pat. No. 6,528,228; the disclosures of which are all incorporated herein by reference.
Although conventional developers and methods of processing work well with imaged thermally imageable multi-layer elements that have been imaged using conventional halftone screening, unacceptable variations in dot percentages are observed throughout the developer loading cycle when the imageable elements have been imaged using stochastic screening. The resultant tone, images are inconsistent and liable to fatten with plates processed later in the developer loading cycle. Thus, a need exists for a method for processing imageable elements that have been imaged using stochastic screening that does not produce unacceptable variations in dot percentages throughout the developer loading cycle.